Reactor for carrying out an exothermic reaction

ABSTRACT

The present invention relates to a process for carrying out an exothermic reaction in the presence of a solid catalyst in a three-phase slurry reactor which includes a slurry zone and a freeboard zone. In the slurry zone, the catalyst is kept in suspension in a slurry liquid and in the freeboard zone, a liquid reflux is maintained to remove catalyst from the freeboard zone and, preferably, recycling the catalyst to the slurry zone. The present invention also relates to a three-phase slurry reactor adapted to the process.

FIELD OF THE INVENTION

The present invention relates to a process for carrying out anexothermic reaction in the presence of a solid catalyst in a three-phaseslurry reactor. According to a further aspect, the present inventionrelates to a three-phase slurry reactor for carrying out an exothermicreaction.

BACKGROUND OF THE INVENTION

Three-phase slurry reactors are well known to those skilled in the art.In operation, the reactor comprises a slurry zone and a freeboard zone.In the slurry zone solid catalyst particles are kept in suspension in aliquid. The liquid serves as heat-transfer medium. The mixture ofcatalyst particles and liquid is commonly referred to as slurry. One ormore gaseous reactants bubble through the slurry zone. The freeboardzone located above the slurry zone contains substantially no slurry andserves as a disengagement zone between slurry, and gaseous products andreactants.

The catalyst particles are typically kept in suspension by stirring oragitation by a mechanical device or, preferably, by an upward gas and/orliquid velocity.

Although substantially all catalyst particles are present in the slurryzone, a proportion of the catalyst particles escape from the slurry zoneinto the freeboard zone and may stick to the reactor wall or internalsin the freeboard zone. In the absence of liquid heat-transfer medium,but in the presence of unreacted gaseous reactants, the said catalystparticles continue to catalyse the exothermic reaction. In this way,local hot spots are created which may damage the reactor vessel and/orinternals.

Accordingly, it would be desirable to be able to remove catalystparticles efficiently from the freeboard zone.

For the purposes of this specification the term catalyst particles isintended as reference to catalyst particles per se and/or any finesthereof

SUMMARY OF THE INVENTION

Therefore, the present invention relates to a process for carrying outan exothermic reaction in the presence of solid catalyst particles in athree-phase slurry reactor comprising a slurry zone and a freeboardzone, in which slurry zone the catalyst particles are kept in suspensionin a slurry liquid, which freeboard zone contains catalyst particlesescaped from the slurry zone, and in which freeboard zone a liquidreflux is maintained to remove the catalyst particles from the freeboardzone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a vertical cross-section through athree-phase slurry reactor.

FIG. 2 schematically depicts a horizontal cross-section through thethree-phase slurry reactor of FIG. 1.

FIG. 3 schematically depicts a detail of a catalyst trap of thethree-phase reactor of FIG. 1.

FIG. 4 schematically depicts a corrugated trap for catalyst particles.

DETAILED DESCRIPTION

The process of the present invention provides for carrying out anexothermic reaction in the presence of solid catalyst particles in athree-phase slurry reactor. The reactor comprises a slurry zone and afreeboard zone. Catalyst particles are kept in suspension in a slurryliquid in the slurry zone and catalyst particles escaped from the slurryzone enter the freeboard zone. A liquid reflux is maintained in thefreeboard zone and this liquid reflux removes the catalyst particlesfrom the freeboard zone. Preferably, the catalyst particles in thefreeboard zone are recycled to the slurry zone by means of the liquidreflux.

The liquid reflux can be generated and maintained by spraying liquidinto the freeboard zone from an external source.

The liquid reflux is typically inert, that is the liquid reflux is not areactant for the exothermic reaction and substantially does not react toother products in the process.

In one preferred embodiment, the liquid reflux from the external sourceis a part of the slurry liquid which is withdrawn from the slurry zone.Following separation of the said liquid from the solid particles bymeans known to those skilled in the art, a part of the said liquid isintroduced into the freeboard zone.

According to another preferred embodiment of the invention, theexothermic reaction produces at least some gaseous products, whichgaseous products are capable of at least partly condensing at atemperature between the reaction temperature in the top part of theslurry zone and 50° C. below the said reaction temperature, and theliquid reflux is generated and maintained by at least partly condensingthe gaseous product in the freeboard zone.

Optionally, a combination of the above methods is employed to maintainthe liquid reflux.

The gaseous products may at least partly be condensed by means known tothose skilled in the art. Thus, in one embodiment the gaseous productsare at least partly condensed by external cooling of the wallsurrounding the freeboard zone, typically the reactor wall.

According to another embodiment, the gaseous products are at leastpartly condensed by allowing more leakage of heat from the freeboardzone of the reactor to the atmosphere, than from the slurry zone. Thiscan suitably be achieved by less thermal insulation in the reactor wallsurrounding the freeboard zone, relative to the reactor wall surroundingthe slurry zone.

According to yet another embodiment, the gaseous products are at leastpartly condensed by cooling means in the freeboard zone. A variety ofknown cooling means may be applied, including indirect cooling meanssuch as cooling coils.

However, a disadvantage of indirect cooling means, such as coolingcoils, present in the freeboard zone is that in this way the volume ofthe freeboard zone occupied by internals is relatively high. Thus, thechance that a catalyst particle escaping from the slurry zone sticks toan internal in the freeboard zone, is high as well. It will beappreciated that according to a preferred embodiment, the volume of thefreeboard zone occupied by internals is minimised.

Accordingly, in one preferred embodiment, the cooling in the freeboardzone is achieved by injection of a relatively cold gas, typically aninert gas. More preferably, cooling is achieved by injection of a liquidwhich vaporises under conditions prevailing in the freeboard zone. Thus,in this embodiment, the cooling means in the freeboard zone typicallycomprises gas or liquid injection means.

The cooling means in the freeboard zone is preferably controllableindependent from the cooling means present in the slurry zone.

The slurry zone can be cooled by direct or indirect cooling means. Forthe purposes of this specification, direct cooling means refers to thosemeans where the cooling medium is in direct contact with the slurry inthe slurry zone. Indirect cooling means refers to those means where thecooling medium is not in direct contact with the slurry in the slurryzone. An example of the latter is an arrangement of cooling tubesimmersed in the slurry. Preferably, the slurry zone is cooled byindirect cooling means.

It will be appreciated that in order to minimise the volume of internalspresent in the freeboard zone, preferably any indirect cooling meansused to cool the slurry zone, hereinafter indirect slurry cooling means,substantially do not extend into the freeboard zone.

Preferably, the average temperature in the freeboard zone is decreasedto a temperature which is up to 50° C. lower than the temperature in thetop of the slurry zone. The temperature in the top of the slurry zone istypically the average temperature prevailing at about 5 to 15 cm belowthe interface between the slurry zone and the freeboard zone. Morepreferably, the decrease is up to 30° C.

The temperature in the freeboard zone is preferably decreased by atleast 5° C., more preferably at least 10° C., relative to thetemperature in the top of the slurry zone.

It will however be understood by those skilled in the art that thedesired temperature decrease depends on a variety of factors such as thequantity of condensing product at a certain temperature; the amount ofcatalyst particles, or fines thereof, which is present in the freeboardzone; the complexity of, and volume occupied by, internals in thefreeboard zone; and the average particle size of catalyst particles orfines present in the freeboard zone. Thus, it will be appreciated, itmay sometimes be preferred to decrease the temperature more or less thanthe preferred ranges given above.

The average particle size of the catalyst particles may vary betweenwide limits, depending inter alia on the type of slurry zone regime.Typically, the average particle size may range from 1 μm to 2 mm,preferably from 1 μm to 1 mm.

If the average particle size is greater than 100μm, and the particlesare not kept in suspension by a mechanical device, the slurry zoneregime is commonly referred to as an ebullating bed regime. Preferably,the average particle size in an ebullating bed regime is less than 600μm, more preferably in the range from 100 to 400 μm. It will beappreciated that in general the larger the particle size of a particle,the smaller the chance that particle escapes from the slurry zone intothe freeboard zone. Thus, if an ebullating bed regime is employed,primarily fines of catalyst particles will escape to the freeboard zone.

If the average particle size is at most 100 μm, and the particles arenot kept in suspension by a mechanical device, the slurry zone regime iscommonly referred to as a slurry phase regime. Preferably, the averageparticle size in a slurry phase regime is more than 5 μm more preferablyin the range from 10 to 75 μm.

If the particles are kept in suspension by a mechanical device, theslurry zone regime is commonly referred to as stirred tank regime. Itwill be appreciated that in principle any average particle size withinthe above ranges can be applied. Preferably, the average particle sizeis kept in the range from 1 to 200 μm.

The concentration of catalyst particles present in the slurry may rangefrom 5 to 45% by volume, preferably, from 10 to 35% by volume. It may bedesired to add in addition other particles to the slurry, as set out infor example European patent application publication No. 0 450 859. Thetotal concentration of solid particles in the slurry is typically notmore than 50% by volume, preferably not more than 45% by volume.

Suitable slurry liquids are known to those skilled in the art.Typically, at least a part of the slurry liquid is a reaction product ofthe exothermic reaction. Preferably, the slurry liquid is substantiallycompletely a reaction product.

The exothermic reaction is a reaction which is carried out in thepresence of a solid catalyst, and which is capable of being carried outin a three-phase slurry reactor. Typically, at least one of thereactants of the exothermic reaction is gaseous. Examples of exothermicreactions include hydrogenation reactions, hydroformylation, alkanolsynthesis, the preparation of aromatic urethanes using carbon monoxide,Kölbel-Engelhardt synthesis, polyolefin synthesis, and Fischer-Tropschsynthesis. According to a preferred embodiment of the present invention,the exothermic reaction is a Fischer-Tropsch synthesis reaction.

The Fischer-Tropsch synthesis is well known to those skilled in the artand involves synthesis of hydrocarbons from a gaseous mixture ofhydrogen and carbon monoxide, by contacting that mixture at reactionconditions with a Fischer-Tropsch catalyst.

Products of the Fischer-Tropsch synthesis may range from methane toheavy paraffinic waxes. Preferably, the production of methane isminimised and a substantial portion of the hydrocarbons produced have acarbon chain length of at least 5 carbon atoms. Preferably, the amountof C5+ hydrocarbons is at least 60% by weight of the total product, morepreferably, at least 70% by weight, even more preferably at least 80% byweight, most preferably at least 85% by weight.

Fischer-Tropsch catalysts are known in the art, and typically include aGroup VIII metal component, preferably cobalt, iron and/or ruthenium,more preferably cobalt. Typically, the catalysts comprise a catalystcarrier. The catalyst carrier is preferably porous, such as a porousinorganic refractory oxide, more preferably alumina, silica, titania,zirconia or mixtures thereof.

The optimum amount of catalytically active metal present on the carrierdepends inter alia on the specific catalytically active metal.Typically, the amount of cobalt present in the catalyst may range from 1to 100 parts by weight per 100 parts by weight of carrier material,preferably from 10 to 50 parts by weight per 100 parts by weight ofcarrier material.

The catalytically active metal may be present in the catalyst togetherwith one or more metal promoters or co-catalysts. The promoters may bepresent as metals or as the metal oxide, depending upon the particularpromoter concerned. Suitable promoters include oxides of metals fromGroups IIA, IIIB, IVB, VB, VIB and/or VIIB of the Periodic Table, oxidesof the lanthanides and/or the actinides. Preferably, the catalystcomprises at least one oxide of an element in Group IVB, VB and/or VIIBof the Periodic Table, in particular titanium, zirconium, manganeseand/or vanadium. As an alternative or in addition to the metal oxidepromoter, the catalyst may comprise a metal promoter selected fromGroups VIIB and/or VIII of the Periodic Table. Preferred metal promotersinclude rhenium, platinum and palladium.

A most suitable catalyst comprises cobalt as the catalytically activemetal and zirconium as a promoter. Another most suitable catalystcomprises cobalt as the catalytically active metal and manganese and/orvanadium as a promoter.

The promoter, if present in the catalyst, is typically present in anamount of from 0.1 to 60 parts by weight per 100 parts by weight ofcarrier material, preferably from 0.5 to 40 parts by weight per 100parts by weight of carrier material. It will however be appreciated thatthe optimum amount of promoter may vary for the respective elementswhich act as promoter. If the catalyst comprises cobalt as thecatalytically active metal and manganese and/or vanadium as promoter,the cobalt: (manganese +vanadium) atomic ratio is advantageously atleast 12:1.

The Fischer-Tropsch synthesis is preferably carried out at a temperaturein the range from 125 to 350° C., more preferably 175 to 275° C., mostpreferably 200 to 260° C. The pressure preferably ranges from 5 to 150bar abs., more preferably from 5 to 80 bar abs.

Hydrogen and carbon monoxide (synthesis gas) is typically fed to thethreephase slurry reactor at a molar ratio in the range from 0.4 to 2.5.Preferably, the hydrogen to carbon monoxide molar ratio is in the rangefrom 1.0 to 2.5.

The gaseous hourly space velocity may vary within wide ranges and istypically in the range from 1500 to 10000 Nl/l/h, preferably in therange from 2500 to 7500 Nl/l/h.

The Fischer-Tropsch synthesis is preferably carried out in a slurryphase regime or an ebullating bed regime, wherein the catalyst particlesare kept in suspension by an upward superficial gas and/or liquidvelocity.

It will be understood that the skilled person is capable to select themost appropriate conditions for a specific reactor configuration andreaction regime.

Preferably, the superficial gas velocity of the synthesis gas is in therange from 0.5 to 50 cm/sec, more preferably in the range from 5 to 35cm/sec.

Typically, the superficial liquid velocity is kept in the range from0.001 to 4.0 cm/sec, including liquid production. It will be appreciatedthat the preferred range may depend on the preferred mode of operation.According to one preferred embodiment, the superficial liquid velocityis kept in the range from 0.005 to 1.0 cm/sec.

As outlined hereinabove, preferably the volume of the freeboard zoneoccupied by internals is minimised. In this way, the chance that acatalyst particle escaping from the slurry zone sticks to an internal inthe freeboard zone, is minimised as well.

However, it may be preferred that the freeboard zone contains meansspecifically designed to trap catalyst particles. Such means e.g. may beused for protecting parts of the freeboard zone which are difficult toclean with a liquid reflux or otherwise, for example outlet means forgases.

Typically, the means to trap catalyst particles will allow relativelyeasy passage of gases whereas catalyst particles, and/or fines thereof,and any liquid droplets entrained with such gases are trapped. Further,the means to trap catalyst particles will allow relatively easy removalof catalyst particles, and liquid droplets, by a liquid reflux.

The means to trap catalyst particles typically comprises one or moremeans which divert the passage of gases. Preferably, the gases areforced to follow a tortuous path. Thus, any catalyst particles collidewith the means to divert the passage of gases. A liquid reflux ismaintained to remove the said catalyst particles.

According to a preferred embodiment the means to trap catalystparticles, hereinafter also referred to as the trap, comprises one ormore, in particular a plurality of, corrugated plates. Each corrugatedplate contains at least one corrugation, preferably a plurality ofcorrugations. The corrugations define crests and troughs on thecorrugated plate. Typically, the corrugated plates are placedsubstantially vertical in the freeboard zone, and preferablysubstantially parallel to the overall direction of flow of the gasesthrough the trap. The crests of the corrugations, however, are placed insuch direction to the overall direction of flow of the gases through thetrap, that the gases are forced to follow a tortuous path through thetrap.

The degree of tortuosity may be defined as the ratio between the actualpath length through the means to trap catalyst particles and ahypothetical shortest path length along a straight line. The said ratiois typically greater than 1:1, preferably at least 1.1:1, morepreferably at least 1.2:1. Typically, the said ratio is not greater than2:1, preferably not greater than 1.5:1.

The angle between the direction of the crests and the overall directionof flow of the gases through the trap is typically at least 30°,preferably at least 60°, more preferably substantially 90°.

The flow of gases in the freeboard zone is normally substantiallyvertical. According to one preferred embodiment, the means to trapcatalyst particles is arranged such that the overall direction of flowof gases through the trap is substantially vertical as well. This allowsa relatively simple construction. It will however be appreciated that adisadvantage of a substantially vertical (upward) flow of gases throughthe trap is that removal of catalyst with a substantially vertical(downward) liquid reflux may be difficult at relatively high gasvelocities.

Therefore, alternatively, the trap is arranged such that the directionof flow of gases through the trap makes an angle of less than 180°, butmore than 0°, with the direction of flow of the liquid reflux.Preferably the angle ranges from 30°to 150°, in particular the angle issubstantially 90°.

Thus, according to another preferred embodiment, the direction of flowof gas through the means to trap catalyst particles is substantiallyhorizontal, and the liquid reflux is substantial vertical. As will beset out in more detail hereinafter, in particular in this embodiment itis preferred to transport catalyst to the slurry zone or to arejuvenation zone inside or outside the reactor vessel, by means of atube which does not allow entrance of gases in the freeboard zone.

The liquid reflux over the means to trap catalyst particles may begenerated and maintained in the same way as described above. Preferably,the trap is cooled to generate a reflux of condensed gaseous product.Typically, the trap comprises cooling means. Thus, corrugated plates maybe connected with one or more cooling tubes. Alternatively, the liquidreflux over the trap is maintained by spraying liquid from an externalsource over the trap.

The corrugations on the corrugated plates may be shaped in a largenumber of ways. Thus, for example, the corrugations may have the shapeof a sine wave, a saw-tooth wave, a triangular wave, a half- orfull-wave rectified sine wave or combinations thereof. The corrugationspreferably do not have the shape of a square wave.

For ease of manufacture, preferably, the shape of the corrugations issubstantially constant across a corrugated plate, and the corrugatedplates preferably have substantially the same size and shape.

Typically, the corrugated plates are arranged substantially in parallel.The space between the plates defines the passage for the gases.Preferably, the corrugated plates are arranged such that the width ofthe passage remains substantially constant.

The space between two corrugated plates is preferably in the range from1 to 10 mm, more preferably from 2 to 5 mm.

The pressure drop between gas inlet and gas outlet of the trap istypically less than 2 bar, preferably less than 0.1 bar. Generally, thepressure drop will be more than 0.01 bar.

According to a further aspect, the present invention relates to a threephase slurry reactor for carrying out exothermic reactions in thepresence of a catalyst, comprising reactant inlet means and productoutlet means a slurry zone equipped with slurry cooling means, and afreeboard zone, wherein the reactor is adapted to maintain a liquidreflux in the freeboard zone.

Typically, the three-phase slurry reactor is specifically adapted to theprocess of the present invention. Thus, it will be understood by thoseskilled in the art that preferred embodiments discussed in relation tothe process are also preferred embodiments with respect to thethree-phase slurry reactor.

Without wishing to be restricted to a particular embodiment, theinvention will now be set out in more detail with reference to FIGS. 1to 4.

In more detail, FIG. 1 depicts a three-phase slurry reactor 1,comprising a reactor wall 2 enclosing a slurry zone 3 and a freeboardzone 5. A sheet 6 defines the bottom end of the slurry zone 3. Dashedline 7 depicts the interface between the slurry zone 3 and the freeboardzone 5.

The reactor 1 is equipped with gas inlet means 60 and gas outlet means70, slurry inlet means 80 and slurry outlet means 90. Sheet 6 allowspassage of gas from gas inlet means 60 to the slurry zone 3.

The slurry zone 3 is equipped with cooling tubes 22, which cooling tubesdo not extend into the freeboard zone 5.

The freeboard zone 5 is equipped with means to trap catalyst particles(trap) 10. The trap 10 contains a plurality of substantially verticalcorrugates plates. The corrugated plates are arranged substantiallyparallel to each other, and substantially parallel to the overalldirection of flow of gasses through trap 10. The corrugated plates maybe contained in a circular housing, but typically, the corrugated platesare contained in two or more, preferably four or six, discreterectangular housings, as depicted in FIG. 2.

Sheet 11 forces gases to flow substantially horizontal through the trap10. Sheet 11 and trap 10 are cooled by cooling tubes as shown in FIGS. 2and 3. In operation, catalyst particles are removed from sheet 11 andtrap 10 by a liquid reflux.

Catalyst particles removed from trap 10 by a liquid reflux, are returnedto the slurry zone 3 by tube 12. The outlet of tube 12 is immersed inthe slurry zone 3.

Turning now to FIG. 2, reference number 2 depicts the reactor wall of athree-phase slurry reactor 1. Six traps for catalyst particles 10 havebeen mounted on sheet 11. Each trap 10 contains a gas inlet 13 and a gasoutlet 14. Gas outlet 14 is in fluid communication with hole 15 in sheet11. Gas entering trap 10 is forced to follow a tortuous path along aplurality of corrugated plates 16. To generate a liquid reflux,corrugated plates 16 are cooled by means of cooling tubes 20. Coolingtubes 20 connect to a header (not shown) in the zone above sheet 11 (seeFIG. 3), and, preferably, is independent from the cooling system in theslurry zone 3.

Turning now to FIG. 3, reference numbers which correspond to referencenumbers in FIG. 1 and/or 2 have the same meaning. Thus, FIG. 3 depicts atrap for catalyst particles 10. Catalyst particles removed from trap 10by a liquid reflux, are returned to the slurry zone 3 by tube 12. Due tothe pressure drop over the trap 10 between gas inlet 13 and gas outlet14, the interface level between slurry zone 3 and freeboard zone 5 intube 12 will be somewhat higher than the level in the reactor, asindicated by dashed line 7.

To generate a liquid reflux, corrugated plates 16 (as shown in FIG. 2)are cooled by means of cooling tubes 20. The cooling tubes are typicallyU-shaped. One end of the cooling tube 20 connects in fluid communicationto coolant inlet tube 25, whereas the other end of cooling tube 20connects in fluid communication to coolant outlet tube 30.

Coolant inlet tube 25 connects in fluid communication to coolant inletdistribution ring 35, and coolant outlet tube 30 connects in fluidcommunication to coolant outlet distribution ring 40. Distribution ring35 connects in fluid communication to inlet means for introducingcoolant into the reactor (not shown) and distribution ring 40 connectsin fluid communication to outlet means for removing coolant from thereactor (not shown).

Suitable coolants are known to those skilled in the art. A preferredcoolant is water and/or steam.

FIG. 4 schematically depicts a corrugated plate 50 for use in a trap 10for catalyst particles as shown in FIGS. 1 to 3. The plurality ofcorrugations on corrugated plate 50 define troughs 51 and crests 52.

The arrow in FIG. 4 indicates the overall direction of flow of gasesthrough trap 10. The corrugated plate 50 is placed substantiallyparallel to the overall direction of flow of gases through the trap,whereas the crests are placed substantially perpendicular to that flow.It will be appreciated that the crests of a plurality of stackedcorrugated plates 50 force the gases to follow a tortuous path throughthe trap.

What is claimed is:
 1. A three-phase slurry reactor for carrying outexothermic reactions in the presence of a catalyst in the form ofcatalyst particles, comprising a reactor wall enclosing a slurry zoneadjacent to a freeboard zone, a reactant inlet means allowing reactantto enter the reactor before the slurry zone, and a product outlet meansallowing product to exit the reactor after the freeboard zone; whereinsaid slurry zone is equipped with a slurry zone cooling means; whereinsaid freeboard zone contains means to trap catalyst particles, saidmeans to trap catalyst particles comprising one or more corrugatedplates, and a means to maintain a liquid reflux in a said freeboardzone, said means to maintain a liquid reflux comprising a cooling meansto cool said means to trap catalyst particles, said liquid reflux havinga characteristic of removing the trapped catalyst particles from saidfreeboard zone.
 2. A three-phase slurry reactor as claimed in claim 1,wherein said one or more corrugated plates each contain a plurality ofcorrugations each having a crest, which corrugated plates are arrangedsubstantially vertical and substantially parallel to an overalldirection of flow of gases through the plurality of corrugated plates,and wherein during operation the crests of the corrugations force thegases to follow a tortuous path through the corrugated plates.